A drainage basin is an area of land where water from rain drains downhill into a body of water, such as a lake or the sea. The drainage basin acts like a funnel, collecting all the water within the area covered by the basin and channelling it into a waterway.

Terminology for the study of rivers Drainage basin - the area of land drained by a river and its tributaries. Catchment area - the area within the drainage basin. Watershed - the edge of highland surrounding a drainage basin. It marks the boundary between two drainage basins. Source - The beginning or start of a river. Confluence - the point at which two rivers or streams join. Tributary - a stream or smaller river which joins a larger stream or river. Mouth - the point where the river comes to the end, usually when entering a sea.

Do you know your drainage basin terminology?

The Long Profile of River from Source to Mouth

PROCESSES

Erosion Processes – how the river wears away its channel Remember H.A.C.A H ydraulic Action – the sheer force of the flowing water on the bed and banks. A brasion– stones and pebbles carried by the river wear away the river channel ( sandpaper effect ) C orrosion - river water dissolves calcium carbonate in chalk and limestone. A ttrition – stones and pebbles carried by river smash into each other, breaking them into smaller pieces and making them rounder.

Transportation - Rivers pick up and carry material as they flow downstream. A river may transport material in four different ways: Solution - minerals are dissolved in the water and carried along in solution. Suspension - fine light material is carried along in the water. Saltation - small pebbles and stones are bounced along the river bed. Traction - large boulders and rocks are rolled along the river bed. Floatation

Deposition When a river loses energy, it will drop or deposit some of the material (load) it is carrying. Deposition may take place when a river enters an area of shallow water or when the volume of water decreases - for example, after a flood or during times of drought. Deposition is common towards the end of a river's journey, at the mouth. Deposition at the mouth of a river can form deltas - for example, the Mississippi Delta.

The Upper Course of a River

River Tay – Upper Course

Middle course of River The river is eroding back, or truncating the spurs to form a wider flat valley floor The combination oneroding on the outer bank and depositing on the inner bank causes the meanders to travel across the flood plain leaving it covered with alluvial silt.

River Tay – Middle course

Lower Course of a river. Transportation and deposition are most important as the river meanders across a wide flood plain. Depositional features include wide flood plain,levees, braiding and deltas.

(6.) Rapids

Deposition A river drops some of its load when either its volume or its speed decreases eg when it enters a flat or gently sloping plain or enters a lake or the sea. Material transported or deposited by a river is called alluvium.

FORMATION OF MEANDERS Cut bank Slip off slope

Flood plain This is an area of flat land found on either side of a river. This usually becomes wider as the river nears its mouth. The flood plain is the area of land which is flooded when a river overflows its banks. In this area layers of alluvium are laid down. The soils are thus thick and fertile and the land is flat.

Deltas are found at the mouth of large rivers - for example, the Mississippi. A delta is formed when the river deposits its material faster than the sea can remove it. There are three main types of delta, named after the shape they create: Three main types of delta Arcuate, cuspate and Birds foot eg Mississippi River Delta Arcuate or fan-shaped - the land around the river mouth arches out into the sea and the river splits many times on the way to the sea, creating a fan effect. Cuspate - the land around the mouth of the river juts out arrow-like into the sea. Bird's foot - the river splits on the way to the sea, each part of the river juts out into the sea, rather like a bird's foot.

Flooding – this is caused when heavy rainfall reaches the river channel too quickly and the channel cannot cope with such a large volume of water. The increased risk of Flooding is usually due to a combination of factors. If there is…. High water table or ground saturated Steep sided impermeable drainage basin Land largely built upon, so water fed directly by drains Exposed farmland, so soil becomes compacted. Woodland and natural groundcover cleared. Houses built on floodplain …………..The heavy rain will run straight to the river How can this be prevented…..?

MEDC case study: causes and effects of flooding in Boscastle, UK (2004) Causes Over 60 mm of rainfall (typically a month's rainfall) fell in two hours. The ground was already saturated due to the previous two weeks of above average rainfall. The drainage basin has many steep slopes, and has areas of impermeable slate causing rapid surface run-off. Boscastle is at the confluence (where tributaries meet) of three rivers - Valency, Jordan, and Paradise. A large quantity of water all arrived within a short space of time causing the rivers to overflow. The flooding coincided with a high tide, making the impact worse. Effects Homes, businesses and cars belonging to more than 1,000 people were swept away. Income from tourism was lost. This had an impact on livelihoods and the local economy. There were vast numbers of subsequent insurance claims. No lives were lost, partly due to the rapid response of the emergency services.

Causes of Flooding January/February 2014 Heavy rainfall, high winds and tidal surges. Increased speed of jet stream has brought a steady flow of depressions across the Atlantic bringing high winds and heavy rainfall for two months Rainfall highest on record for January, double average This is combined with unusually high tides which bring high tidal waters up the Bristol channel and into the Rivers Parrett and Tone Clay ground saturated – increased run off. So rivers burst their banks and flood What are the Somerset Levels and Why are they prone to flooding? The Somerset Levels and Moors is a sparsely populated coastal plain and wetland area of central Somerset, in South West England, running south from the Mendip Hills to the Lowland area that is crossed by rivers that drain surrounding hills. These rivers swell from the tidal surge that comes up the Bristol Channel.coastal plainwetlandSomersetSouth West EnglandMendip Hills The land has been drained for agriculture But farmers are planting crops not Pasture which leaves land exposed to Run off aggravating flooding. Outline the impact of the floods in Somerset The village of Muchelney, cut off since Jan 3 River Parrett burst its banks Existing flood defences breached 25 square miles flooded. Village of Moorland evacuated 150+ homes flooded Homes and cars damaged/destroyed Farmland submerged for so long crops and pasture ruined Huge financial cost – not everyone sufficiently well insured so calls for government compensation.. What have been the responses to the floods Evacuation of Homes by emergency services and livestock by RSPCA Volunteers and fire brigade helping to transport people to and from stranded villages. Calls for Government action – compensation For environment agency to manage environment better eg dredging Dutch pumps employed to pump water off land back into already full rivers What measures could be taken to prevent the floods in the future? Dredging the rivers Parrett and Tone? To increase capacity and reduce overflow Managed flooding upstream – natural barriers using fallen trees to allow flooding of less populated land upstream and reduce floods on levels Pay farmers to allow their land to be flooded and take pressure off settlements.